Bump MSRV to 1.81 #1057

Job	Run time
minimal-versions / minimal-versions	42s
set-msrv / set-msrv	4s
build (1.81.0, thumbv7em-none-eabi)	19s
build (1.81.0, wasm32-unknown-unknown)	23s
build (stable, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	15s
test (1.81.0)	37s
test (stable)	24s
	3m 2s

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